Assessment of advection dispersion through excavation damaged zone in sedimentary rock by in situ tracer tests

Takeda, Masaki ; Ishii, Eiichi   

Uunderstanding nuclide transport characteristics in the EDZ of disposal and access tunnels is an essential issue in the safety assessment of geological disposal of high-level radioactive waste. Although tracer tests are effective in evaluating the transport of nuclides in rock masses, the transport properties of EDZ in sedimentary rock, to our best knowledge, have not been investigated by in situ tracer tests. The authors conducted cross-hole tracer tests on EDZ fractures at the Horonobe Underground Research Laboratory to evaluate their longitudinal dispersibility. One-dimensional advection-dispersion analyses based on the tracer test data were performed, and the longitudinal dispersibility was estimated to be 0.12 m for the test scale of 4.2 m. This longitudinal dispersibility is 1/100 to 1/10 of the test scale, comparable with the empirical relationship between the test scale and longitudinal dispersibility for natural fractures and rock matrices. The series of tracer tests and analyses reported in this paper demonstrate that advection-dispersion occurs also in EDZ fractures similarly to natural fractures and rock matrices, and that longitudinal dispersibility in EDZ fractures can be assessed also by conventional in situ tracer test methods.